1. Field of the Invention
The present invention relates to a phase demodulator and a portable telephone apparatus which demodulate phase-modulated received signals.
2. Related Art
Many types of QPSK demodulation systems are known which are used in radio communication devices such as portable telephones. Most of them adopt an orthogonal demodulation method.
There are various types of orthogonal demodulation systems. One prevailing system is a superheterodyne system. In this system, a carrier-frequency signal that has passed through a bandpass filter and has been amplified with a low noise amplifier (LNA) is mixed with a signal from a local oscillator by a down converter to convert the signal into an intermediate frequency (IF) signal with a lower frequency.
The IF signal is inputted to a bandpass filter to select a targeted channel frequency and is amplified by a valuable gain amplifier (VGA). An orthogonal demodulator performs demodulation processes for two systems orthogonal to each other (different in phase by 90 degrees). In each system, the received signal is mixed with an output of a local oscillator for generating IF signal to convert the received signal into a baseband frequency. Subsequently the frequency-converted signal passes through a lowpass filter to be digitized by an A/D converter, and a demodulation process is conducted.
Recently, a direct conversion system has been popular in a GSM cellular phone system and the like. In the direct conversion system, the signals of a carrier frequency that have passed through a bandpass filter and been amplified by the LNA are branched into two portions by the orthogonal demodulator. Those branched signals are mixed with the orthogonal output of the local oscillator and directly converted into the baseband frequency. Subsequently a targeted channel frequency is selected by a lowpass filter, amplified with the VGA, digitized by the A/D converter, and then the demodulation process is conducted (refer to Japanese Patent Publication No. 3052614).
Both the superhererodyne and the direct conversion systems have a problem in that enormous numbers of circuits are necessary, i.e. two types of demodulation processes for the received signal by using the orthogonal demodulator, the local oscillator for generating the high frequency reference signal, the mixer and the down converter for converting the high frequency received signal to the IF signal, the bandpass filer for selecting the received channel, the A/D converter for conducting A/D conversion, the demodulator and the like are necessary.
A system, as a future wireless system, has been studied which directly conducts A/D converter by over-sampling in a state of a carrier frequency, selects the channel signal by digital process and conducts the demodulation process. There is a problem in that the use of such a system will be unpractical in near future, because the A/D conversion performed by over-sampling at a very high carrier frequency such as 800 MHz to 2 GHz requires a very high speed A/D converter, and power consumption will increase to more than several tens of Watts or more.